1. Field of the Invention
The invention relates generally to subsurface characterization of geologic formations. More specifically, the invention relates to using sensors mounted outside wellbore casing in crosswell electromagnetic measurement techniques.
2. Background Art
Subsurface characterization of earth formations is an important aspect of drilling, for example, oil and gas wells. Subsurface characterization may help identify, among other factors, the structure and fluid content of geologic formations penetrated by a wellbore. The geologic formations surrounding the wellbore may contain, for example, hydrocarbon products that are the target of drilling operations. Knowledge of the formation characteristics is important to hydrocarbon recovery.
Geologic formations that form a hydrocarbon reservoir contain a network of interconnected fluid paths, or xe2x80x9cpore spaces,xe2x80x9d in which, for example, hydrocarbons, water, etc., are present in liquid and/or gaseous form. To determine the hydrocarbon content in the pore spaces, knowledge of characteristics such as the porosity and permeability of the geologic formations penetrated by the wellbore is desirable.
Information about the geologic formations and about reservoir characteristics promotes efficient development and management of hydrocarbon resources. Reservoir characteristics include, among others, resistivity of the geologic formation containing hydrocarbons. The resistivity of geologic formations is generally related to porosity, permeability, and fluid content of the reservoir. Because hydrocarbons are generally electrically insulating and most formation water is electrically conductive, formation resistivity (or conductivity) measurements are a valuable tool in determining the hydrocarbon content of reservoirs. Moreover, formation resistivity measurements may be used to monitor changes in reservoir hydrocarbon content during production of hydrocarbons.
Formation resistivity measurements are often made with wireline conveyed measurement while drilling (MWD) and logging while drilling (LWD) tools. However, wireline MWD and LWD resistivity tools typically only measure formation resistivity proximate the individual wellbore in which they are operated. As a result, there have been several attempts to determine the resistivity of geologic formations surrounding and between adjacent wellbores drilled into the geologic formations of interest. For example, measurement of formation resistivity between adjacent wellbores using a low frequency electromagnetic system is discussed in two articles: Crosshole electromagnetic tomography: A new technology for oil field characterization, The Leading Edge, March 1995, by Wilt et al.; and Crosshole electromagnetic tomography: System design considerations and field results, Society of Exploration Geophysics, Vol. 60, No. 3, 1995, by Wilt et al.
FIG. 1 shows an example of a system used to measure formation resistivity between two wellbores. A transmitter T is located in one wellbore and consists of a coil CT having multi-turn horizontal loop (vertical solenoid) of N1 turns and an effective cross section AT. The multi-turn horizontal loop carries an alternating current IT at a frequency of f0 Hz. In free space, the multi-turn horizontal loop produces a time varying magnetic field B0. The magnetic field B0 is proportional to a magnetic moment MT of the transmitter T and to a geometric factor k1. The magnetic moment MT of the transmitter T can be defined as follows:
MT=NTITAT.xe2x80x83xe2x80x83(1)
In free space, the magnetic field B0 can be defined as follows:
xe2x80x83B0=k1MT.xe2x80x83xe2x80x83(2)
The geometric factor k1 is a function of a spatial location and orientation of a component of the magnetic field B0 measured by a receiver R.
The receiver R is located some distance from the transmitter T and is typically disposed in a different wellbore. The receiver R typically includes a loop of wire (e.g., a coil CR having NR turns wound about a core of high magnetic permeability metal such as ferrite). A time-varying magnetic field BR sensed by the receiver R, having a frequency f0, creates an induced voltage VR in the coil CR which is proportional to BR, the frequency f0, the number of turns of wire NR, an effective cross-sectional area of the coil AR, and an effective magnetic permeability xcexcR of the coil CR. From the foregoing, VR can be defined as follows:
VR=f0BRNRARxcexcR.xe2x80x83xe2x80x83(3)
By simplifying equation (3), VR may be written as follows:
VR=kRBR.xe2x80x83xe2x80x83(4)
where kR=f0NRARxcexcR. The product of ARxcexcR is difficult to calculate. To accurately determine ARxcexcR, CR is calibrated in a known magnetic field and at a known frequency to determine an exact value for kR. Thereafter, the magnetic field BR sensed by the receiver R is related directly to the measured voltage VR by the following equation:                               B          R                ⁢                  xe2x80x83                =                  xe2x80x83                ⁢                                            V              R                                      k              R                                .                                    (        5        )            
When a system such as this is placed in a conductive geologic formation, the time varying magnetic field B0 produces an electromotive force (emf) in the geologic formation which in turn drives a current therein, shown schematically as L1 in FIG. 1. The current L1 is proportional to the conductivity of the geologic formation and the flow of the current L1 is generally concentric about the longitudinal axis of the wellbore. The magnetic field proximate the wellbore is a result of the free space field B0, called the primary magnetic field, and the field produced by the current L1 is called the secondary magnetic field.
The current L1 is typically out of phase with respect to the transmitter current IT. At very low frequencies, where the inductive reactance of the surrounding formation is small, the induced current L1 is proportional to dB/dt and is, consequently, 90xc2x0 out of phase with respect to IT. As the frequency increases, the inductive reactance increases and the phase difference increases.
The secondary magnetic field detected by the receiver R is caused by the induced current L1 and also has a phase shift so that the total magnetic field at the receiver R is complex in nature. The total magnetic field has a component BR in-phase with the transmitter current IT (referred to as the real component) and a component B1 phase shifted by 90xc2x0 (referred to as the imaginary or quadrature component). The values of the real BR and quadrature B1 components of the magnetic field at a given frequency and geometric configuration uniquely specify the electrical resistivity of a homogenous formation penetrated by the wellbores. In a nonhomogeneous geologic formation, the complex magnetic field is generally measured at a succession of points along the longitudinal axis of the receiver wellbore for each of a succession of transmitter locations. The multiplicity of T-R locations suffices to determine the nonhomogeneous resistivity between the wellbores as described in the references listed below.
In general, nonhomogeneous distribution of electrical resistivity in a geologic formation is determined through a process called inversion, which is well described in Audio-frequency electromagnetic tomography in 2-D, Geophysics, Vol. 58, No. 4, 1993, by Zhou et al.; Electromagnetic conductivity imaging with an iterative born inversion, IEEE Transactions on Geoscience and Remote Sensing, Vol. 31, No. 4, 1993, by Alumbaugh et al.; An approach to nonlinear inversion with applications to cross-well EM tomogaphy, 63rd Annual International Meeting, Society of Exploration Geophysics, Expanded Abstracts, 1993, by Torres-Verdin et al.; and Crosswell electromagnetic inversion using integral and differential equations, Geophysics, Vol. 60, No. 3,. 1995, by Newman. The inversion process has been used to determine resistivity in the vicinity of a single wellbore or between spaced-apart wellbores wells and is described in detail in Crosswell electromagnetic tomography: System design considerations and field results, Geophysics, Vol. 60, No. 3, 1995, by Wilt et al.; Theoretical and practical considerations for crosswell electromagnetic tomography assuming a cylindrical geometry, Geophysics, Vol. 60, No. 3, by Alumbaugh and Wilt; and 3D EM imaging from a single borehole: a numerical feasibility study, 1998, by Alumbaugh and Wilt.
One embodiment of the inversion process comprises assigning resistivities to a multitude of xe2x80x9ccellsxe2x80x9d or elements of the space surrounding, or between, wellbores. The resistivities are systematically varied until the results from the cellular model of the formation most closely match observed data taken with the field transmitter receiver system described herein. In another embodiment, a more specific model of the formation is assumed using geological, well log, or other geophysical data. The parameters of this model (e.g., resistivity distribution, formation shape, layer thickness, etc.) are varied until the numerical results from the model most closely match the measured data. In another embodiment, direct images of the distribution of resistivity may be obtained following the principles of diffusion tomography as described in Audio-frequency electromagnetic tomography in 2-D, Geophysics, Vol. 58, No. 4, 1993, by Zhou et al. In yet another method, multifrequency electromagnetic data is transformed into a mathematically defined wave field domain and the data is processed following the procedures of seismic tomography. These means of interpreting the electromagnetic data are included here to illustrate the fact that electromagnetic methods are of practical use in determining the resistivity of geological formations.
Measurements of resistivity distribution between wellbores are usually made before and during extraction of hydrocarbons to detect hydrocarbon reservoirs and to monitor changes in reservoir resistivity as hydrocarbons are extracted. The system of FIG. 1 operates where the wellbore does not include conductive casing therein. Wellbores, however, typically include conductive liners or casing, typically steel, in order to preserve the physical integrity of the wellbore and the surrounding formations during hydrocarbon extraction and/or further drilling operations. Because typical casing is highly electrically conductive, magnetic fields intended to be introduced into the formation are strongly attenuated by the casing. Casing is very difficult (if not impossible) to remove from the wellbore once installed. As a result, the system shown above in FIG. 1 does not facilitate analysis of a hydrocarbon reservoir once conductive casing has been installed.
The problems presented by conductive casing in a wellbore of interest are described by Augustin et al. in A Theoretical Study of Surface-To-Borehole Electromagnetic Logging in Cased Holes, Geophysics Vol. 54, No. 1, 1989; Uchida et al. in Effect of A Steel Casing on Crosshole EM Measurements, SEG Annual Meeting, Texas, 1991; and Wu et al., in Influence of Steel Casing on Electromagnetic Signals, Geophysics, Vol. 59, No. 3, 1994. From these references, it may be observed that the casing conductivity may be modeled as an additional xe2x80x9cshorted wirexe2x80x9d closely coupled to the transmitter T, shown schematically as L2 in FIG. 1.
A net or effective magnetic moment Meff of the transmitter/conductive casing combination is controlled by the inductive coupling therebetween. Physically, the resistivity of the conductive casing is very low while the inductance is relatively high. This results in an induced current in the conductive casing that is approximately 180xc2x0 out of phase with the transmitter current IT. The induced current is of opposite polarity with respect to the transmitter current IT but of almost the same moment. Therefore, the magnetic field external to the conductive casing is greatly reduced. In effect, the conductive liner xe2x80x9cshieldsxe2x80x9d the transmitter T from the receiver R positioned outside of the conductive casing. Any magnetic field outside the casing is produced by the difference in current, and hence moment, between the transmitter T and the conductive casing.
Because the induced moment in the casing is large and nearly equal to the transmitter moment, small changes in the properties of the casing produce large fractional changes in the effective moment. In practice, casing is known to be nonhomogenous (e.g., there are variations in casing diameter, thickness, permeability, and conductivity that may be caused by, for example, manufacturing/processing procedures or by corrosion/stress/temperature processes after installation in a wellbore). The central issue for the electromagnetic methods described above for non-cased, or open, wellbores is that the fields from the transmitter are severely attenuated in a cased well and that the net moment is highly variable as the transmitter traverses the length (e.g., the depth) of the well. Without precise knowledge of casing properties, it is difficult to distinguish between external field variations caused by the casing and variations produced by the formation.
A magnetic field sensor positioned within a cased wellbore experiences an analogous situation. The magnetic field to be detected induces current flowing concentrically with the receiver coil, and the induced current tends to reduce the magnetic field within the casing. The measurable magnetic field is consequently highly attenuated, and the measurement is highly influenced by the variations in attenuation caused by the variation in casing properties described above. Often, the design criteria for a crosswell survey of a cased wellbore reduces the magnetic field signal to a level that is undetectable by standard receivers. Moreover, the variance in conductivity, permeability, and thickness along a longitudinal axis of a length of casing makes it difficult to determine an attenuation factor at any selected point. The inability to determine an attenuation factor at a selected point may cause errors in field measurements that are not easily corrected.
A prior attempt to overcome this limitation involves inclusion of a separate small-scale transmitter-receiver within the cased wellbore to measure the casing properties. The measured casing properties are then used to correct the measured crosswell data. See, e.g., Lee et al., Electromagnetic Method For Analyzing The Property of Steel Casing, Lawrence Berkeley National Laboratories, Report 41525, February, 1998.
Another prior attempt to correct for the magnetic field attenuation in a cased wellbore includes positioning a monitor receiver adjacent to the transmitter in the cased wellbore. In this manner, an attempt is made to predict the attenuation sensed by, for example, a receiver located in an adjacent wellbore. This method is disclosed in U.S. patent application Ser. No. 09/290,156, filed Apr. 12, 1999, entitled Method and Apparatus for Measuring Characteristics of Geologic Formations, and assigned to the assignee of the present invention.
In U.S. patent application Ser. No. 09/394,852, filed Sep. 13, 1999, entitled An Electromagnetic Induction Method and Apparatus For The Measurement of the Electrical Resistivity of Geologic Formations Surrounding Boreholes Cased with A conductive Liner, and assigned to the present assignee, a method for measuring formation resistivity adjacent to and between cased wellbores using low frequency ( less than 200 Hz) multiturn solenoidal coils within cased wellbores is disclosed. Specifically, the method disclosed therein allows measurement of the resistivity of geologic formations proximate a wellbore encased with a conductive, or metallic, casing made from materials such as steel. The method includes taking appropriate ratios of measured fields either inside or outside of the metallic casing so that attenuation due to the casing is practically canceled.
Measurements with the aforementioned method are difficult to perform once production from the well has begun and production tubing has been run from the surface to the producing zone. The production tubing leaves little or no room for the electromagnetic measurement system to move in the well. Repeated measurements to monitor production or enhanced recovery processes as a result require repeated removal and reinsertion of the production tubing. This is a costly operation, and it is clear that a permanent monitoring system, on the outside of the casing, would be more cost effective.
What is needed, therefore, is a cross-well measurement technique that provides accurate resistivity measurements of geologic formations without requiring detailed information concerning the electrical and magnetic properties of a liner disposed in the wells, and that does not reduce production efficiency of the wells.
In one aspect, the invention comprises a method for determining characteristics of geologic formations between wellbores. The method comprises activating at least one transmitter to generate a first magnetic field, the at least one transmitter disposed about an external surface of a conductive liner at a selected depth in a first wellbore. A formation magnetic field induced by the first magnetic field is detected with at least one receiver disposed about an external surface of a conductive liner at a selected depth in a second wellbore. A characteristic of the geologic formation is determined from the detected formation magnetic field.
In another aspect, the invention comprises a method for determining characteristics of geologic formations between wellbores. The method comprises activating a first of a plurality of axially spaced transmitters to generate a first magnetic field, the plurality of transmitters disposed about an external surface of a conductive liner at selected depths in a first wellbore. A first formation magnetic field induced by the first magnetic field is detected with a first of a plurality of axially spaced receivers, the plurality of receivers disposed about an external surface of a conductive liner at selected depths in a second wellbore. The first formation magnetic field induced by the first magnetic field is then detected with a second of the plurality of receivers. A first amplitude ratio is calculated from the first formation magnetic fields detected by the first and second of the plurality of receivers.
The method further comprises activating a second of the plurality of transmitters to generate a second magnetic field. A second formation magnetic field induced by the second magnetic field is detected with the first of the plurality of receivers. The second formation magnetic field induced by the second magnetic field is then detected with the second of the plurality of receivers. A second amplitude ratio is calculated from the second formation magnetic fields detected by the first and second of the plurality of receivers. A third amplitude ratio is then calculated from the first amplitude ratio and the second amplitude ratio, and a characteristic of the geologic formation is determined from the third amplitude ratio.
In another aspect, the invention comprises a method for determining characteristics of geologic formations between wellbores. The method comprises activating a first transmitter at a first selected depth to generate a first magnetic field, the first transmitter disposed on a drilling tool disposed at a selected depth in a first wellbore. A first formation magnetic field induced by the first magnetic field is detected with a first of a plurality of axially spaced receivers, the plurality of receivers disposed about an external surface of a conductive liner at selected depths in a second wellbore. The first formation magnetic field induced by the first magnetic field is detected with a second of the plurality of receivers. A first amplitude ratio is calculated from the first formation magnetic fields detected by the first and second of the plurality of receivers.
In another aspect, the invention comprises a method for telemetering data between wellbores. The method comprises activating a first transmitter to generate a first magnetic field, the first transmitter disposed on a drilling tool disposed in a first wellbore. A formation magnetic field induced by the first magnetic field is detected with at least one receiver, the at least one receiver disposed about an external surface of a conductive liner at a selected depth in a second wellbore. A drilling tool characteristic is determined from the detected formation magnetic field.
In another aspect, the invention comprises a system for determining characteristics of geologic formations between conductively lined wellbores. The system comprises at least two axially spaced apart electromagnetic transmitters positioned at selected depths in a first wellbore and disposed about an external surface of a conductive lining. At least two axially spaced apart electromagnetic receivers are positioned at selected depths in a second wellbore and disposed about an external surface of a conductive lining. At least one surface control station is operatively coupled to the at least two transmitters and the at least two receivers, and the at least one surface control station is adapted to selectively activate each of the at least two electromagnetic transmitters to generate first magnetic fields and to selectively activate each of the at least two electromagnetic receivers to detect formation magnetic fields induced by the first magnetic fields.
In another aspect, the invention comprises a system for determining characteristics of geologic formations between conductively lined wellbores. The system comprises a plurality of axially spaced electromagnetic transmitters disposed about an external surface of a conductive liner at selected depths in a first wellbore, and the plurality of electromagnetic transmitters are adapted to generate first magnetic fields. A plurality of axially spaced electromagnetic receivers are disposed about an external surface of a conductive liner at selected depths in a second wellbore, and the plurality of electromagnetic receivers are adapted to detect formation magnetic fields induced by the first magnetic fields. The system also comprises means for calculating an amplitude ratio from the detected formation magnetic fields, and means for determining a characteristic of the geologic formation from the amplitude ratio.
In another aspect, the invention comprises a system for determining characteristics of geologic formations proximate a conductively lined wellbore. The system comprises a plurality of axially spaced electromagnetic transmitters disposed about an external surface of a conductive liner at selected depths in a wellbore, and the plurality of electromagnetic transmitters are adapted to generate first magnetic fields. A plurality of axially spaced electromagnetic receivers are disposed about an external surface of a conductive liner at selected depths in the wellbore, and the plurality of electromagnetic receivers adapted to detect formation magnetic fields induced by the first magnetic fields. The system also comprises means for calculating an amplitude ratio from the detected formation magnetic fields, and means for determining a characteristic of the geologic formation from the amplitude ratio.